Before us is Mishnah 5 in chapter 4 of Tractate Shevi'is, which deals with cutting branches from a tree: in which ways one may protect the place of the cut, and when the cutting itself is considered work forbidden in the Sabbatical year.
"HaMavki'a b'zayis":
"HaMavki'a b'zayis" - one who cuts branches from an olive tree and wishes to protect the place of the cut, which is like an open wound in the tree.
"Lo yechapehu b'afar" - he may not cover the place of the cut with earth, since once the earth becomes wet it turns into a muddy substance that protects the tree. Such a covering benefits the tree and improves it, and work of this kind is forbidden in the Sabbatical year.
"Aval mechasehu ba'avanim uvakash" - he may protect the place of the cut with a covering of stones or straw, which merely shields it and adds no benefit to the tree.
"HaKotzetz koros shikmah":
The law is similar for one who cuts the beams of a sycamore tree, which were used for building: "lo yechapehu b'afar" - he should not cover the place of the cut with earth, "aval mechasehu b'even uvakash" - but he may cover it with stones or straw.
"Ein kotztzin besulas shikmah bashevi'is":
A further law was stated regarding the sycamore tree, whose branches were cut for use as building timber: it is forbidden to cut a "besulas shikmah" - a tree that was never cut before, and the present cutting is the first one it undergoes. The reason: "mipnei shehi avodah" - the first cutting is considered work that benefits the tree and improves it, and it is therefore forbidden in the Sabbatical year.
The view of Rabbi Yehudah:
Rabbi Yehudah distinguishes between the ways of cutting: "kedarkah" - one who cuts in the usual manner is forbidden. And what is the usual manner? They would normally cut below ten handbreadths, but slightly above the surface of the ground. If he cut in a manner that is not the usual way, he may do so even in the Sabbatical year, and there are two ways of doing this:
"O magbiah asarah tefachim" - he cuts above ten handbreadths, which is not the usual manner.
"O gomah me'al ha'aretz" - the complete opposite: he cuts until nothing remains protruding from the ground, and this too is not the usual manner.
To summarize: In this Mishnah we learned that one does not cover the place of the cut on an olive tree or on sycamore beams with earth, because such a covering improves the tree, but one may protect it with stones and straw. We also learned that one does not cut a "besulas shikmah" in the Sabbatical year, because its first cutting is considered work that benefits the tree, and according to Rabbi Yehudah the prohibition applies specifically to one who cuts in the usual manner: if he cut above ten handbreadths, or right up against the ground, it is permitted.